Answer:

It can be used as a mnemonic device (memory tool) for which of a telephone's number buttons the letters of the alphabet are grouped together on:

(1=No letters, therefore "1" is not in the sequence.)

2=ABC
2 A)djustable B)aseball C)aps

3=DEF
3 D)ragons E)xhaling F)ire

4=GHI
4 H)orses G)alloping I)n

5=JKL
5 L)ively K)angaroos J)umping

6=MNO
6 O)range N)ut M)uffins

7=PQRS
7 S)eas P)irate R)um Q)uest

8=TUV
8 V)egetables U)nited T)ogether

9=WXYZ
9 Z)ebras W)ere X)-rayed Y)esterday

(0=No letters)

Note: What matters is that the initial letters of the words of each phrase comprise the letters that belong together in each group. Therefore, the sequence of the initial letters for each group's phrase doesn't necessarily have to be in alphabetical order.

The title is meant to be a humourous indirect reference to the answer in that both the numbers on the telephone and fingers which are used to press their buttons may be referred to as digits.